Process for the conversion of aromatic hydrocarbons

ABSTRACT

A process for the conversion of aromatic hydrocarbons is disclosed which is especially useful for reaction of an alkylating agent, preferably propylene, with an aromatic hydrocarbon. Novel feature is use of a catalyst system comprising TiCl 4  and a Group III-A metal oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved process for the conversionof an aromatic hydrocarbon in the presence of a titanium tetrachloridecatalyst system.

The invention will be described with reference to alkylation, i.e., thesynthesis of cumene by alkylation of benzene with propylene in thepresence of the catalyst, but will find use in alkylaromatictransalkylation and isomerization as well.

2. Description of the Prior Art

Conversion of aromatic hydrocarbons is well known in industry. Some ofthe aromatic conversion reactions which occur include alkylation ofaromatic hydrocarbons with an alkylating agent such as an olefin,disproportionation of alkylaromatic hydrocarbons and isomerization ofalkylaromatic hydrocarbons such as xylenes.

Of special interest, has been the propylation of benzene to cumene.Cumene is used for the production of phenol and acetone. Cumene is alsodehydrogenated to form methylstyrene, in a process similar to that usedto convert ethylbenzene to styrene. Cumene is also used as a blendingcomponent in aviation gasoline because of its high octane number. Theconsumption of cumene in the U.S.A. was about 350,000 metric tons in1968. Of this total, 94% was used for the production of phenol oracetone.

It is well known that cumene can be synthesized from benzene andpropylene using a catalyst of AlCl₃, SPA or BF₃. SOA is a generallyaccepted abreviation for solid phosphoric acid catalyst, or phosphoricacid which is adsorbed on kieselguhr or other support.

AlCl₃ is a very popular alkylation catalyst, because of its highactivity. Unfortunately, the catalyst operates as a slurry or sludgewhich is messy to handle on a commercial scale, and also is corrosive.The highly reactive nature of this Friedel-Crafts metal halide catalyst,AlCl₃, is desirable when attempting to alkylate benzene with ethylene,because less active catalyst systems do not work. However, foralkylation with propylene such highly reactive systems are notnecessary.

The AlCl₃ catalyst, although not equivalent to a Ziegler-Natta catalyst,is similar, at least for some olefin reactions. AlCl₃ will promotepolymerization of both ethylene and propylene, and alkylation of benzenewith both ethylene and propylene. These catalysts also promotetransalkylation of alkyl groups. This is in contrast to slightly lessactive systems, such as SPA catalyst, which catalyzes alkylation ofbenzene with propylene, but does not catalyze transalkylationsatisfactorily.

Another highly selective catalyst system has been developed for thealkylation of benzene with olefins. This catalyst comprises borontrifluoride. The boron trifluoride catalyst system is exceptionallyactive and permits operation with dilute olefin streams, but it requiresthe continuous addition of BF₃ to maintain catalyst activity. Highcatalyst activity also leads to oligomerization of olefins, so contacttime of olefins with BF₃ catalyst should be as short as possible. Thiscatalyst is also exceptionally water sensitive, as water not onlydestroys the catalyst, but produces very corrosive solutions whichattack downstream processing units. BF₃ also frequently appears in theproduct, and must be removed therefrom.

Because of the interest in alkylating benzene with olefins, and becauseof the inadequacies of existing catalyst systems, I studied the workthat others had done, and made exhaustive investigations to determine ifit would be possible to find a catalyst which would have the activityand selectivity required to produce an acceptable cumene product, whilemaking maximum use of existing petroleum resources.

A highly active catalyst was sought, to permit operation at lowertemperatures with less utility cost, cost of construction, and tooperate with less catalyst. In new units this would mean smaller, andcheaper reactor vessels, while in existing units it would mean that anincrease in capacity could be obtained by changing catalyst in anexisting reactor vessel, with minor modifications.

High selectivity is necessary, not only to permit operation withfeedstreams which are not 100% pure olefin, but also to maximizeproduction of the desired product, and to minimize production ofpolymerized olefins, or polyalkylated aromatic compounds.

Accordingly, many catalyst systems were studied to find a catalyst withexcellent activity and selectivity, which was not corrosive or destroyedby water.

There has been extensive work done with Ti catalysts, though most workoccurred in conjunction with studies of Ziegler-Natta catalysts. Theclosest prior art known is U.S. Pat. No. 2,381,481 (Class 260-683.15),U.S. Pat. No. 2,951,885 (Class 260-671), U.S. Pat. No. 2,965,686 (Class260-671) and U.S. Pat. No. 3,153,634 (Class 252-429).

In U.S. Pat. No. 2,381,481, preparation and use of a catalyst preparedby treating alumina gel with fluotitanic acid is disclosed. Thiscatalyst is used for polymerization of olefins to heavier hydrocarbons,and also for alkylation of parafins with olefins, the latter whenoperating at high temperatures, between 700° and 900° F or higher. Nomention is made of alkylation of aromatics with olefinic hydrocarbons ortransalkylation of polyalkylbenzenes.

In U.S. Pat. No. 2,951,885, there is disclosed the use of titaniumtrihalide on activated alumina or other activated acidic oxide foralkylation of benzene with olefins. The catalyst is originally atetrachloride, subsequently reduced to the trichloride with an alkalimetal such as sodium, lithium, or potassium. The examples show that thiscatalyst will alkylate benzene with ethylene.

In U.S. Pat. No. 2,965,686 the thrust of the application was to developa titanium subchloride catalyst. In Example II, a reaction betweencumene and propylene was disclosed using titanium tetrachloridecatalyst. The catalyst in Example II was tetrachloride. The catalyst wasprepared by activating alumina by evacuation at a temperature of 600° Cfor an unspecified period of time. The resultant catalyst was then usedin an alkylation reaction for the propylation of cumene to formdiisopropylbenzene. This patent is silent as to the type of aluminawhich was used as the base for the catalyst.

In U.S. Pat. No. 3,153,634, there is disclosed the use of titaniumsubhalides in a polymerization reaction. The patentee is probablydescribing a catalyst for production of solid polymer products. On page3 lines 65-75, the patentee mentions use of benzene as an inert solventto hold dissolved olefins, rather than as a reactant.

Accordingly, work continued on developing an improved process for thecatalytic conversion of aromatic hydrocarbons.

Accordingly, the present invention provides a process for the catalyticconversion of an aromatic hydrocarbon comprising contacting the aromatichydrocarbon with a reactant at aromatic hydrocarbon conversionconditions in the presence of a catalyst system comprising titaniumtetrachloride and alumina wherein the catalyst system is prepared bypassing TiCl₄ vapor over activated alumina at a temperature of 20° to400° C for 1 to 10 hours.

The catalyst used comprises titanium tetrachloride impregnated on anactivated metal oxide which possesses surface hydroxyl groups. Specificexamples of these metal oxides will include the Group III-A metal oxideswhich possess surface hydroxyl groups and which also possess arelatively high surface area such as alumina, gallium oxide, indiumoxide, and thallium oxide. Of these compounds, the preferred substrateis alumina, and especially low density, high surface area aluminas suchas gamma-alumina or, if so desired, eta-alumina.

The apparent bulk density of the alumina may range from about 0.3 toabout 0.7 g/cm³ or higher with a surface area ranging from about 1 toabout 500 m² /g. The alumina may be in any shape, one example of thesubstrate being spheroidal alumina which is prepared by the conventionaland commercial oil-drop method as described in U.S. Pat. No. 2,610,314.In addition, it is also contemplated within the scope of this inventionthat the alumina may be treated to provide greater physical stability,one type of treatment being to impregnate the gamma-alumina with acompound such as barium nitrate, which, upon calcination, is convertedinto barium oxide. The latter compound will then, as hereinbefore setforth, provide greater physical stability for the alumina. It is alsocontemplated within the scope of this invention that a commercialgamma-alumina may also be used as the support. However, since thiscommercial gamma-alumina could contain an excessive amount of waterwhich would consume an excess of titanium tetrahalide without anybeneficial effect on the catalyst, in the preferred embodiment of thisinvention the commercial gamma-alumina is subjected to a predrying stepby heating to a temperature in the range of from about 400° to about550° C under an inert gas or hydrogen flow for a period of about 1 toabout 8 hours.

In order to achieve the maximum activity of the metal oxide support, itis necessary to avoid severe drying of said support. For example, thedrying of alumina at temperatures in excess of about 600° such as 650°,under vacuum, will seriously deplete the alumina of the hydroxyl groupspresent thereon. This severe drying step will not only remove the waterwhich is absorbed on the alumina, but will also remove the aforesaidsurface hydroxyl groups which are essential to make an active catalyst,said surface hydroxyl groups reacting with the titanium component of thetitanium tetrachloride.

The gamma-alumina which has been predried according to the aboveparagraph is then placed in an appropriate apparatus which may comprisea flask, tube, etc., and a gas mixture of nitrogen and titaniumtetrachloride which has been prepared by bubbling nitrogen gas throughthe liquid titanium tetrachloride at room temperature is passed over thegamma-alumina at temperatures of 25 to 135. Thereafter the temperatureis increased to 550 or more. The passage of the nitrogen-titaniumtetrachloride mixture over the alumina is effected about 0.5 to 10 hoursor more, the time being dependent upon the amount of gamma-alumina whichis present and the flow rate of the titanium tetrachloride-nitrogen gasmixture. It is preferred to pass the titanium tetrachloride the gaseousmixture over the support at a temperature of about 25° to about 135°.The temperature is then raised to 250 or a desired temperature, eithergradually or in a series of steps. The preferred temperature for theheat treatment of this resulting composite is from about 135 to about550, however, it will be dependent on the temperature which is used inthe aromatic hydrocarbon conversion process. Generally, it is preferredthat the temperature which is used in the treatment of the composite beequal to, or higher than, the aromatic hydrocarbon conversiontemperature. Thereafter the temperature is maintained at this point anda stream of nitrogen is passed over the catalyst composite for anadditional period which may range from about 1 to 10 hours. At the endof this time, the finished catalyst is then sealed under an inertatmosphere such as argon, helium, nitrogen, etc., prior to being used.

Alternatively, the catalyst may be prepared by forming a solution oftitanium tetrachloride in a polar, non-aqueous organic solvent andimpregnating the alumina. Thereafter the impregnated alumina is thentreated under a nitrogen flow at temperatures in the range hereinbeforeset forth. After subjecting the impregnated substrate to thesetemperatures for a predetermined period of time, the finished catalystis also recovered and maintained under an inert atmosphere until usethereof.

It is believed that the important factor in the thermal treating stepsis the temperature, rather than the total time, as long as the totalperiod for thermal treatment is reasonably long; around 5 or 6 hours. Itis believed that the activity of the catalyst is effected by the thermaltreatments because the thermal treatments desorb water molecules fromthe catalyst surface. Water can compete for active sites with thereactants, thus water is to some extent a catalyst poison. However, ifcatalyst deactivation occurs due to water adsorption, the catalyst canbe regenerated by appropriate further thermal treatment under inert gasflow. Any regenerative thermal treatments should probably approximatethose of the orignal thermal treatments, in that heating which is toorapid, or to too high a temperature, may cause hydrolysis of the TiCl₄component on the catalyst, which would reduce catalyst activity. Anotherdanger of a rapid high temperature catalyst regeneration would be theformation of corrosive gases and liquids due to rapid evolution of H₂ Ovapor and chlorine compounds.

It is also contemplated within the scope of this invention that thecatalyst system hereinbefore described may, if so desired, be compositedon a solid support. The preferred solid supports which may be utilizedcomprise high surface area inert compounds, some representative examplesof which will include silica, magnesia or mixtures of silica with otherinorganic oxides such as silica-zirconia, silica-thoria,silica-magnesia-zirconia, etc., charcoal, coal, diatomaceous earths andclays such as fuller's earth, bentonite, montmorillonite, kieselguhr,etc. It is to be understood that these compounds will act only assupports for the catalyst system and will not enter into the catalyticactivity of the composite. The catalyst system comprising titaniumtetrachloride composited on the Group IIIA metal oxide and the inertsupport may be composited in any manner known in the art such as byimpregnation, deposition, rolling, mixing, etc.

In addition, it is also to be considered within the scope of thisinvention that one or more promoters may be added to the catalystsystem. It is believed that use of one or more promoters selected fromthe metals of Group VIB or Group VIII of the Periodic Table may bebeneficial to the practice of the present invention.

At least about 0.5 weight percent titanium, on an elemental basis, isbelieved necessary for a significant amount of reaction to occur. Theupper limit on titanium is believed to be about 20 wt. %.

When it is desired to use the catalyst system in an alkylaromaticisomerization process, then alkylaromatic isomerization reactionconditions should be used. Reaction conditions are disclosed in U.S.Pat. No. 3,637,881 (Class 260-668a), the teachings of which areincorporated by reference. When it is desired to use the catalyst systemof the present invention for alkylaromatic transalkylation thenappropriate reaction conditions should also be used. These are disclosedin U.S. Pat. No. 3,720,726 (Class 260-672t), the teachings of which areincorporated by reference. Reaction conditions for the alkylation ofaromatic hydrocarbons will be discussed in detail in a latter part ofthis specification.

The catalyst may be disposed in a reactor vessel as a fixed fluidized ormoving bed of catalyst. The reactants may contact the catalyst inupflow, downflow or crossflow fashion, though upflow of reactants over afixed bed of catalyst is preferred.

The liquid hourly space velocity in the reactor may range from 0.1 to20. Because catalyst of the present invention is very active for thealkylation reaction, significantly higher space velocities are possiblethan when using some prior art catalysts, e.g., SPA. To some extent, theliquid hourly space velocity is related to temperature in the reactionzone, in general, a higher LHSV will require higher temperatureoperation.

The ratios of reactants and other reaction conditions which occur whenalkylating benzene with light olefins, and preferably are basicallythose well known in the art. Pressures may range from 1 to 100atmospheres, or even higher. It is desirable to maintain pressures highenough to have a liquid phase in the reaction zone. Although it ispossible to operate at very high pressure, little advantage is gainedthereby, in fact, an increase in pressure seems to have a harmfuleffect. Preferred pressure seems to be around 20 to 60 atm, with anoptimum pressure of about 35 atm.

Temperature effects both the conversion and selectivity of the reaction.Temperature may range between ambient and 250. At very low temperatures,the catalyst is not sufficiently active to permit the desired reactionto proceed at a satisfactory rate. At very high temperatures, it isbelieved that the catalyst may be damged, by formation of carbonaceousmaterials on the catalyst.

If the reaction is kinetically controlled, an increase in temperatureshould increase the rate of reaction. As a general statement, this istrue, but the temperature dependence may not be as large as expected, ifthe reaction is limited by mass transport of reactants and products toand from the catalyst surface. Preferred operating temperature isprobably about 100 to 200 C.

The catalyst may be disposed in a reactor vessel as a fixed, fluidizedor moving bed of catalyst. The reactants may contact the catalyst inupflow, downflow or crossflow fashion, through upflow of reactants overa fixed bed of catalyst is preferred.

The liquid hourly space velocity in the reactor may range from 0.1 to20. However, higher LHSV is possible depending on the desired conversionlevel of propylene. Because catalyst of the present invention isbelieved very active for the alkylation reaction, significantly higherspace velocities should be possible than when using some prior artcatalysts, e.g., SPA. To some extent, the liquid hourly space velocityis related to temperature in the reaction zone, in general a higher LHSVwill require higher temperature operation.

EXAMPLE I

In this example a catalyst was prepared by predrying 125 cc ofgamma-alumina at a temperature of 550° C for a period of 6 hours under aflow of 2000 cc/min. nitrogen gas. Thereafter a gaseous mixture ofnitrogen and titanium tetrachloride which was prepared by bubblingnitrogen gas through liquid titanium was passed over the gamma-aluminaat a temperature of 75° C for a period of 40 minutes. The flow rate ofnitrogen was 2000 cc/min. At the end of this time, the temperature wasincreased to 250° C while maintaining the nitrogen-titaniumtetrachloride vapor flow over the gamma-alumina. The nitrogen-titaniumtetrachloride flow was discontinued and the catalyst composite wastreated with a nitrogen flow for a period of 4.4 hours while maintainingthe temperature at 250° C. At the end of this period, the catalyst wasanalyzed and found to contain 2.17% titanium and 4.86% chlorine.

ILLUSTRATIVE EMBODIMENT I

The reaction contemplated is alkylation of benzene with propylene.Catalyst is maintained as a fixed bed, of 50 cc volume. Reactants arepassed upflow over the catalyst bed. Benzene is dried by circulating itover high surface area sodium. Pure propylene is dried by passing itover type 4-A molecular sieves. Benzene and propylene are mixed togetherand charged to the reactor. The reaction is carried out at 120° to 245°C, 1 to 3 LHSV, and at 25 to 55 atmospheres pressure. The reactor isstarted up full of liquid benzene and then the mixture of propylene andbenzene added. It is believed that if propylene alone is charged, oreven propylene and benzene charged simultaneously, high molecular weightpolymer may form. Using the conditions indicated above, a high yield ofcumene is expected.

I claim as my invention:
 1. A process for the alkylation of an aromatichydrocarbon which comprises contacting the aromatic hydrocarbon with analkylating agent at aromatic alkylation conditions in the presence of acatalyst system consisting essentially of titanium tetrachloride on anactivated Group III-A metal oxide having surface hydroxyl groups, saidcatalyst system having been prepared by passing TiCl₄ vapor with aninert gas over said metal oxide at a temperature of 20° to 400° C. for 1to 10 hours and thereafter thermally treating the resultant TiCl₄-containing oxide in an inert atmosphere at a temperature of from about135° to about 550° C.
 2. Process of claim 1 wherein the aromatichydrocarbon is selected from the group consisting of benzene, toluene,ethylbenzene and xylene isomers.
 3. Process of claim 1 wherein the metaloxide is alumina.
 4. Process of claim 1 wherein the alumina isgamma-alumina.
 5. Process of claim 1 wherein the alumina is activated bytreatment with N₂ at 300° to 550° C for 1 to 5 hours.
 6. Process ofclaim 1 wherein the catalyst contains, on an elemental basis, about 0.5to 20 wt. % titanium.